a) Field of the Invention
The present invention relates to a head-mounted virtual display apparatus (VDA) based on a cross-cavity optical configuration with an unobstructed forward field of view. More particularly, a near-eye light deflecting element (LDE) located in the peripheral field of view provides xe2x80x9clook towardxe2x80x9d access to an inset magnified image of a miniature display. Active and passive alignment means, including articulating connections and image warping electronics, allow correction of geometric distortion arising from folding of the optical train and/or embodiments based on off-axis optical configurations.
b) Description of the Prior Art
The head-mounted display (HMD) field has evolved on a number of fronts over the past 20 years. The earliest development by the military focused on wide field of view (FOV), see-through helmet-mounted displays for aircraft guidance and weapon aiming applications, in which the virtual image overlies the ambient environment. Since then development has included lightweight monocular HMDs for workplace wearable computer systems, binocular HMDs for full-immersion viewing of video and virtual reality applications, and various types of see-through displays for augmented reality applications.
Monocular HMDs are designed to provide access to electronic information while obscuring only a portion of the forward and peripheral fields of view. A typical monocular HMD approach places the display and optics directly in front of one eye, such that the forward FOV of that eye is partially or fully occluded and the peripheral FOVs of one or both eyes is partially occluded. The most common example of this type of monocular HMD is a boom style HMD, in which the viewable element (and often the display) is positioned in front of the face at the end of a cantilever arm. The main advantages of a boom style HMD include its relative simplicity (i.e., its one size fits all nature and minimal number of adjustments) and its construction flexibility, in that it can be added to a pair of spectacles or any head-borne structure, or can be constructed as a stand-alone headset. The disadvantages of a boom style HMD include a physical boundary that extends a distance from the face, occlusion of a portion of the forward FOV, and its suitability primarily for stationary activities due to vibration of the cantilever arm during user motion.
A second monocular HMD approach integrates the virtual display elements, in part or in full, into a pair of spectacles, with the aim of not significantly altering its form or weight. This approach allows the display and optics to be kept closer to the face, thus making it possible to limit the occluded FOV to one eye and, in some cases, to only a small portion of the peripheral FOV. The compact nature of a glasses-mounted display (GMD), however, generally requires a folding of the optical train, which increases the complexity of the construction.
In general, monocular HMDs can be categorized according to whether the optical train is an on-or off-axis configuration. In an on-axis optical configuration, the optical axis of each powered optical element is coincident with the optical train axis or illumination path (with the exception of unpowered LDEs used to xe2x80x9cturn cornersxe2x80x9d). No optics are xe2x80x9ctiltedxe2x80x9d with respect to the optical train axis. Off-axis optical configurations, on the other hand, generally include at least one powered optical element whose optical axis is tilted with respect to the optical train axis. Off-axis optical configurations allow more compact constructions but suffer from higher levels of aberrations.
Monocular HMDs can be further categorized according to the nature of the magnification system, of which there are two basic types: simple and compound magnification systems. A simple magnification system (or simple magnifier) is a single stage, non-pupil forming magnification system (i.e., a magnification system that does not form a real exit pupil), which is composed of either a positive refractive or reflective element, or multiple adjacent refractive elements with no spacing between them. A compound magnification system, on the other hand, is a pupil forming magnification system typically composed of two or more distinct stages. In a compound magnification system, the stage closest to the object is termed the objective or relay, while the stage viewed by the eye is termed the eyepiece or ocular. In a two stage compound magnification system, the objective forms an xe2x80x9cintermediatexe2x80x9d image (either real or virtual) that is the xe2x80x9cobjectxe2x80x9d projected virtually by the eyepiece. For the purposes of this invention, a third type of magnification systemxe2x80x94termed a compound eyepiecexe2x80x94is defined as one in which multiple refractive and reflective elements (including the eyepiece) are in close proximity to one another with spacing between at least two of the elements. A compound eyepiece is effectively a single stage (pupil-forming) magnification system, which is typically located closer to the eye than it is to the display. Put another way, the distance between the display and the first magnifying element (or the xe2x80x9cobjectivexe2x80x9d) of the system is typically greater than the distance between the first magnifying element and the eyepiece. For a compound magnification system the converse typically holds. For example, consider an HMD with a display located above the eye and a compound eyepiece located below the eye, which is formed from a single block of material and includes three magnifying surfaces: a refractive entrance surface, a reflective intermediate surface and a refractive exit surface. This device includes multiple spaced magnifying elements (so it cannot be categorized as a simple magnification system) and the distance between the entrance and exit surfaces (or the xe2x80x9cobjectivexe2x80x9d and xe2x80x9ceyepiecexe2x80x9d for comparison purposes) is less than the distance between the display and the xe2x80x9cobjectivexe2x80x9d. Thus, the magnifying power is not distributed throughout the optical train like a two stage, compound magnification system.
The design of an HMD involves two generally conflicting aims: (i) achieving a high quality, computer monitor sized virtual image (i.e., a virtual image with a diagonal dimension of at least 10 inches and preferably 15 inches or greater) at a desired apparent image distance (such as a workstation distance of about 24 inches) and (ii) the desire for a compact, lightweight format. One method of balancing these aims is through the use of lightweight, reflective or light deflecting elements (LDEs), such as a mirror constructed from a plastic substrate and a reflective film. In addition, powered and unpowered LDEs may be used to increase magnification (the latter by increasing the optical train path length) and to distribute the weight of the optics more evenly about the head.
A monocular HMD for mobile activities must present a stationary virtual image to the eye during user motion. This requires that the support frame be stably secured to the head and that the display and optics be stably secured to the frame. Taking user comfort into account, the former requirement is best satisfied by a support frame in contact with both ears and the bridge of the nose; while the latter requirement negates the use of a relatively long, thin cantilever arm as the support structure for attaching the eyepiece to the frame, since this type of structure is susceptible to vibration during user motion. For safety and performance reasons, another key requirement for a mobile activity HMD is unobstructed forward vision.
For the purposes of the present invention, the head-mounted display field is further categorized according to: (i) whether the device is suitable for mobile activities; (ii) the optical configuration obstructs normal forward vision; and (iii) whether the optical configuration is a cross-cavity optical configuration (CCOC) or a non-cross-cavity configuration (non-CCOC).
As defined by Geist in U.S. patent application Ser. No. 10/216,958, incorporated herein by reference in its entirety, a cross-cavity optical configuration is an optical configuration in which at least two elements of the optical train lie on opposite sides of the ocular cavity, such that when the system is properly aligned (using articulating alignment means), the light path crosses directly in front of a forward gazing eye. In addition, a mobile activities HMD is defined by Geist as an HMD with an unobstructed forward line-of-sight of at least 35xc2x0 and an unshakeable head-borne mounting (i.e., a head-mounted support in contact with the bridge of the nose and at least two additional areas of the side(s) and/or back of the head, such that the resulting three contact areas provide a stable, unshakeable platform for the optical train). Suitable mobile activities head-mounted supports include, but are not limited to, conventional eyewear, goggles held in place with a strap or headband, and a headset style head-borne support in contact with one ear and/or the side of the head, in addition to the bridge of the nose.
A key factor in compact HMD designs is the level of optical aberrations or image degrading factors. For the purposes of this invention, image degrading factors are divided into two general categories.
The first category of image degrading factors includes all types of geometrical distortions, including those inherent in most off-axis optical configurations. In general, geometric distortion represents the inability of the system to correctly map the shape of the object into image space (i.e., geometrical distortion represent mapping errors). In the case of conventional, symmetric distortion (commonly referred to as barrel and pincushion distortion), the image appears warped (or bowed) inwards or outwards. In the case of keystone distortion, a difference in path length from one area of the object to another results in a trapezoidal shaped image for a nominally rectangular object. Keystone distortion arises in off-axis projection systems and in optical systems when the optical axis of a powered optic is not perpendicular to the plane of the object (e.g., when the magnifying stage is tilted with respect to the display or vice versa). Keystone distortion is inherent in most off-axis HMD optical configurations, as are some higher-order, asymmetric types of geometric distortion.
One of the most detailed analyses to date of geometric distortion in off-axis or non-axially symmetric optical configuration was reported by J. S. Sasian and is entitled Image Plane Tilt in Optical Systems (SPIE No. 1527, Current Developments in Optical System Design and Optical Engineering, 1991), and is incorporated herein in its entirety. A modified form of the Scheimpflug condition is derived for a bilaterally symmetric system
Anu1 tan(xcex8xe2x80x2)xe2x88x92u tan(xcex8)=G+Wimage tilt 
in which An is the coefficient of image anamorphism; u and uxe2x80x2 are the angles of the marginal paraxial ray with respect to the optical axis in the object and image space, respectively; xcex8 and xcex8xe2x80x2 are the tilt angles of the object and image planes, respectively, relative to a plane perpendicular to the optical axis; G is a coefficient associated with the breaking of axial symmetry; and Wimage tilt is a coefficient associated with image plane tilt arising from optical aberrations. For an axially symmetric system with a tilted object plane, An=1 and G=0, and the regular Scheimpflug condition holds. When G is non-zero, tilting of the image plane may occur, even when the object plane is perpendicular to the optical ray axis. A relevant and interesting example with regard to the present invention is that of prism. While G=0 for a prism and a flat mirror, Wimage tilt is non-zero for a prism due to coma (since the stop of the system is not located at the surface of the element), which leads to the familiar fact that the image plane tilt is one-third of the prism angle. As further noted by Sasian, keystone or trapezoidal distortion is closely related to image plane tilt. The coefficient of keystone distortion is defined as:   K  =            m      ⁢                                    tan            ⁡                          (                              θ                f                            )                                -                      tan            ⁡                          (              θ              )                                      f              =                  -                  A          n                    ⁢                                    tan            ⁡                          (                              θ                f                xe2x80x2                            )                                -                      tan            ⁡                          (                              θ                xe2x80x2                            )                                      f            
in which m is the magnification, f is the front focal length, and xcex8f and xcex8xe2x80x2f are the angles of tilt of the front and back-focal planes, respectively.
The purely geometric nature of these types of image degrading factors allow them to be quantified and the display images predistorted (i.e., compensated electronically or computationally) in such a way as to cancel out the geometric distortion generated by the optics. Presently a number of companies offer image warping chips for this purpose. For example, the LEHK-3C display controller from Liesegang Electronics is capable of predistorting images to correct for the aforementioned geometrical distortions. When applicable, this approach is particularly useful in HMD constructions since it allows the number of elements in the optical train to be kept to a minimum.
In practice, however, unless the distortion is of a fixed, unchanging nature, some means of adjusting the position and/or orientation of at least one optical train element is generally required to minimize or eliminate sources of geometric distortion in a multi-user HMD.
The second category of image degrading factors are those that cause a decrease in image sharpness or quality and include chromatic aberrations, astigmatism, coma and spherical aberrations, among others. This category of image degrading factors must be addressed through the use standard optical design techniques (which typically involves using multiple optical elements, surfaces and/or coatings to achieve a desired set of optical parameters, such as image magnification, exit pupil size, exit pupil location, etc.) while maintaining a level of image sharpness acceptable to the eye. For example, the off-axis optical configurations of most wide FOV, see-through HMDs suffer from a higher degree of coma, astigmatism and higher-order asymmetric distortion than a comparable on-axis optical configuration. The predominate image-degrading aberration of most off-axis optical configurations is third-order astigmatism, which, in the case of wide field of view HMDs, is typically minimized through the use of a toroidal reflective eyepiece.
Proper orientation and alignment of the observable virtual image is a key factor in user comfort and extended use of an HMD. Orienting a real image, such as a written document or computer screen, at a comfortable viewing angle is an every day activity. Quantitatively, orientation of the observable virtual image plane (VIP) is defined in terms of angles xcex1 and xcex2 (FIG. 1). Three groups of xcex1 and xcex2 values are pertinent to the present discussion. The first group corresponds to the case when the observable VIP is normal to the optical axis between the eye and the image plane, i.e., when xcex1=xcex2=90xc2x0. This corresponds to the image orientation when viewing an object at optical infinity and, for the purposes of this invention, is termed two-dimensional orthogonality. The second group of values of interest is when xcex2 differs from 90xc2x0 and the image plane is tilted in an undesirable way. The third case of values is an acceptable deviation from two-dimensional orthogonality corresponding to a slight forward or backwards tilting of the observable VIP and is defined herein as one-dimensional orthogonality: xcex2=90xc2x0 and 120xc2x0xe2x89xa7xcex1xe2x89xa770xc2x0. Briefly summarizing, it is not generally acceptable to a viewer for xcex2 to deviate from 90xc2x0, but some deviation from two-dimensional orthogonality may be acceptable (to many users) and may be preferable for certain user specific tasks.
It follows that a mobile activities HMD satisfying two-dimensional orthogonality (or one-dimensional orthogonality with a variable) generally requires one or more moveable/articulating connections to align the optical train with the eye of different users.
A number of boom-style or cantilever arm type HMDs have appeared in the prior art that do not obscure normal forward vision (such as U.S. Pat. No. 4,869,575 disclosed by Kubik) but are not suitable for mobile activities due to vibration of the cantilever arm during user motion. In addition a common disadvantage of this type of HMD is the inability to moveably and independently adjust the near-eye LDE (also commonly referred to herein as the near-eye optic).
Prior art based on a non-CCOC include disclosures by Kurtz (WO 98/29775) and Geist (U.S. patent application Ser. No. 10/216,958). Kurtz discloses a mobile activities HMD based on a non-CCOC, wherein a pair of miniature displays and optical means are positioned above eye level. However, no alignment means-are provided to establish one- or two-dimensional orthogonality (or more commonly referred to herein simply as orthogonality) for different users. Geist provides the alignment means necessary to establish orthogonality for different users, but the alignment means for a non-CCOC differ from those required for a CCOC.
Prior art based on an CCOC include Furness et. al. (U.S. Pat. No. 5,162,828), Heacock et. al. (U.S. Pat. No. 5,539,422), Bettinger (U.S. Pat. No. 4,806,011), Spitzer (U.S. Pat. No. 5,886,822), Holakovszky et al. (U.S. Pat. No. 5,129,716), Wells et. al. (U.S. Pat. No. 5,334,991) and Beadles and Balls (U.S. Pat. No. 5,648,789). Many of the embodiments of these inventions can be classified as mobile activities HMD. However, none of these inventions provide the alignment means necessary to orthogonally align the observable virtual image plane for different users when the near-eye optic is located in the peripheral FOV and normal forward vision is completely unobscured.
The head-mounted virtual display apparatus of Furness et. al. employs a simple magnification system (consisting of either a single aspheric mirror or a single positive lens with a flat mirror for the near-eye LDE) to project a virtual image at a fixed distance from the eye. A pivoting adjustment changes the angle of the near-eye LDE relative to the eye to provide a first alignment means. Furness et. al. does not, however, provide the second alignment means necessary to establish orthogonality.
The head-mounted virtual display apparatus of Heacock et. al. attaches a multi-element or compound eyepiece to a spectacle-type frame. The alignment means provided by Heacock et. al, however, are insufficient to establish orthogonality for different users since the display and eyepiece are not both simultaneously adjustable (i.e., the display is fixed in place while the eyepiece is adjustable). Moreover, Heacock et. al. restricts the location of the near-eye LDE to below eye level.
Bettinger creates a GMD by adding an illumination source and off-axis optical train to a standard pair of spectacles. The key feature of Bettinger is the use of the lens surface as the substrate for a reflective near-eye LDE (i.e., for a concave mirror formed by coating a portion of the spherical lens surface with a reflective material). Bettinger does not provide the moveable connections necessary to orthogonally align the observable VIP for different users. In addition, Bettinger does not allow modification of the conventional spectacle frame form. Moreover, Bettinger does allow the near-eye LDE to be independent of and non-integral with the lens, or permit the use of aspheric lenses. Furthermore, Bettinger does not allow the near-eye LDE to be a flat mirror or allow the curvature of the near-eye LDE to be readily varied, since the construction is limited to standard lens curvatures.
Spitzer fully integrates the optics and electronics for a monocular HMD into a pair of eyeglasses. A transparent or semi-transparent LDE embedded in a spectacle lens is used to superimpose a virtual image on the wearer""s field of view, where a portion of the optical pathway is required to be internally disposed within the lens. Spitzer does not provide means for articulating the near-eye LDE (or an adjacent LDE) as is necessary to orthogonally align the observable VIP for different users. In addition, Spitzer does not allow the optical pathway to be entirely external of the lens, nor allow the near-eye LDE to be independent of and non-integral with the lens.
Holakovszky et al. disclose a stereoscopic, spectacle-type virtual display apparatus in which a pair of relay mirrors (i.e., near-eye optics) are positioned in the normal forward FOV to redirect the light path to the user""s eyes. In addition, Holakovszky et al. does not provide the alignment means necessary to orthogonally align the observable VIP for different users when the near-eye optics are located the normal peripheral FOV. Thus, Holakovszky et al. cannot provide the unobstructed forward vision required for use during mobile activities. Wells et. al. disclose a stereoscopic, spectacle-type virtual display apparatus in which a pair of pivoting (transparent or opaque) relay mirrors, positioned in the normal forward FOV and driven by individual scanning mirror mechanisms, project separate but equivalent scanned images to the user""s eyes. Wells et. al. does not provide the alignment means necessary to locate the near-eye optics in the normal peripheral FOV
Beadles and Balls disclose a spectacle-type virtual display apparatus for superimposing alphanumeric text on the forward FOV (of a user watching a live or video performance) for the purposes of closed captioning. In all embodiments of the invention, a beamsplitter partially obscures the normal forward FOV.
In order to overcome the above-mentioned deficiencies and problems in the prior art, this invention teaches a method of constructing an HMD for mobile activities based on a cross-cavity optical configuration, in which the near-eye optic is located in the peripheral field of view.
1. Objects of the Invention
A general object of this invention is to provide a virtual display apparatus, for temporary or permanent attachment to a head-mounted apparatus, that does not obstruct forward vision and thus is suitable for mobile activities.
Another general object of this invention is to provide a virtual display apparatus for mobile activities of modular construction, with individual and detachable assemblies for the illumination source and optics.
2. Features of the Invention
In keeping with these objects and others that will become apparent hereinafter, one feature of the invention resides, briefly stated, is a virtual display apparatus in which the illumination source is viewed indirectly via a near-eye light deflecting element.
A further feature of the invention resides in a virtual display apparatus with an inset image located anywhere in the peripheral FOV, such that normal forward vision (as defined herein) is unobstructed.
A still further feature of the invention resides in the use of active and passive alignment means, including moveable connections, extended LDEs and image warping electronics, to minimize or eliminate geometric distortion due to tilting of the observable virtual image plane.
A still further feature of the present invention resides in a selection of light deflecting means for the near-eye optic, including spherical and aspherical mirrors, and partially transparent mirrors.
A still further feature of the present invention resides in the use of distinct assemblies for the illumination source, near-eye optic, (adjacent and non-adjacent) folding optics and any additional optics (thus providing modular construction capability).
A still further feature of the present invention resides in freedom to place elements of the virtual display apparatus completely or partially within the boundary of a head-mounted support frame or completely outside the boundary of a head-mounted support frame.
As used herein, the terms magnification or magnifying are sometimes used to denote both magnification and demagnification. Accordingly, the terms magnification and magnifying encompass, and are sometimes used herein to denote, magnification of greater than one, demagnification of less than one and unit magnification. In addition, the terms powered and unpowered are used herein to refer to optical elements with non-zero and zero diopter values, respectively.
As used herein, conventional eyewear refers to all varieties of prescription and non-prescription eyeglasses (or spectacles) including, but not limited to, sunglasses, computer glasses and safety glasses. Common features of conventional eyewear include a structural support frame that uses both ears and the bridge of the nose for support, weight bearing and stabilization during user activity; and individual lenses covering each eye, which are attached and connected to the support frame. The support frame of conventional eyewear is typically comprised of three principal elements: two temples or earpieces, which rest atop the ears and extend from behind the ears to near the temple, and a lens holder, which extends from temple to temple and rests atop the bride of the nose via an integral or removably attached nosepiece or bridge support. The temples of conventional eyewear are typically, but not exclusively, movably attached to the lens holder. Integral or single-piece support frames are also known. In addition, the lens holder of conventional eyewear typically, but not exclusively, includes means for detachably mounting the lenses to the lens holder. Lens/lens holder combinations with the lenses rigidly, but not permanently, affixed to the lens holder are also known, as are integral lens/lens holders.
For the purposes of this invention, the term light deflection means refers to any type of optical element with substantial reflective characteristics. This includes partially and fully reflective mirrors, optical elements based on total internal reflection (such as a non-dispersing, reflecting prism), and holographic optical elements transcribed with reflective properties. The reflective properties of a mirror depend on the nature of the reflective coating applied to the supporting substrate (which may be glass, plastic or other appropriate material). The reflective layer is typically created by depositing a metal coating (such as aluminum or silver) or affixing a reflective polymer film using an adhesive or other standard bonding method. The substrate""s surface contour may take any non-planar or curving form (e.g., a spherical, toroidal or parabolic surface contour).
Image placement refers to changing the apparent distance from the eye of a focused observable virtual image. Image placement plays a key role in minimizing eye (muscle) fatigue and possible user discomfort during extended periods of HMD use. The standard approach to reducing eye fatigue is to place the virtual (or apparent) image at an apparent (or perceived) distance comparable to that of the primary objects in the user""s forward FOV in order to minimize accommodation when the eye switches back and forth between the virtual image and the primary objects. For example, rather than having the virtual image at a standard reading distance of 250 mm, a person working at a computer may wish to perceive the image at a workstation distance of 600 mm to minimize the need for accommodation by the eye when switching between the real image of the computer screen and the inset virtual image of the present invention. This may be accomplished by either fixing the apparent distance based on the primary task of the wearer or by including an adjustment to allow the user to change the apparent distance according to the task at hand.
Furthermore, focusing or focus control refers to the placement of a sharp, resolute virtual image (i.e., an image in which aberrations are sufficiently low to prevent blurring of pixel detail) within the region defined by a user""s near point (i.e., the closest a person can clearly view an object) and far point (i.e., the farthest they can clearly view an object).
It will be understood by one of ordinary skill in the art that when an articulating means is employed to move the near-eye optic (and any underlying support elements) outside the normal peripheral FOV, latching mechanisms may be used to temporarily secure the near-eye optic in its functional and non-functional positions.
It will be further understood by one of ordinary skill in the art that standard techniques for minimizing glare and washout from external and internal sources of illumination, such as anti-reflective coatings, opaque coatings, opaque baffling, opaque housings, etc., may be required.
It will be still further understood by one of ordinary skill in the art that sensors, transducers, and/or microprocessors may be incorporated into any embodiment the present invention.
It will be still further understood by one of ordinary skill in the art that audio/visual accessories, such as an audio speaker, a microphone, a camera, etc., may be incorporated into any embodiment of the present invention.
It will be still further understood by one of ordinary skill in the art that a supplemental means of securing the apparatus to the headxe2x80x94such as an adjustable strap or elastic headbandxe2x80x94may be used to help prevent against slippage and/or dislodging of the head-mounted support during user motion and activity.
Many of the embodiments of the present invention involve off-axis optical configurations, which are susceptible to geometric distortion if care is not taken to xe2x80x9cproperly alignxe2x80x9d the elements of the optical train with the eye. In the most basic construct of the present invention, proper alignment corresponds to the effective centers of the image source assembly, the near-eye optic, the adjacent folding optic, and the pupil of the eye (when directed at the near-eye optic) forming a single optical plane, termed the principal optical plane (POP). In the case of a horizontal POP, establishment of the POP corresponds to setting xcex1=90xc2x0. In other constructs of the invention, one or more non-adjacent folding optics may be used to redirect illumination from the image source assembly to the adjacent folding optic. In such constructs, the first element of the POP is a non-adjacent folding optic. The sections of the optical train leading up to the section constituting the POP are referred to as adjunct optical planes (AOPs). To minimize geometric distortion associated with the AOPs, it is generally preferred that the AOPs be only perpendicular or parallel to the POP. An example of an optical configuration with a single AOP, perpendicular to a horizontal POP has an integral near-eye/adjacent folding optic assembly located at eye level adjacent to the bridge of the nose, a non-adjacent folding optic located at eye level on the opposite side of the ocular cavity, and an image source assembly located below the non-adjacent folding optic next to the cheekbone.
An example of an optical train configuration, according to the present invention, with multiple adjunct optical planes has the image source assembly located below eye level near the cheek bone; a first non-adjacent folding optic located above eye level, directly above the image source assembly; a second non-adjacent folding optic horizontally across from the first non-adjacent folding optic and aligned with the eye (for redirecting a horizontal section of the optical train downwards); an adjacent folding optic located below eye level, directly below the second non-adjacent folding optic; and a near-eye optic located in the general area of a typical bifocal lens (or the reading glass location). The first adjunct optical plane (AOP) is defined by the true and/or effective centers of the real image source, the magnifying stage and first non-adjacent folding optic and is vertically oriented. The second AOP is horizontally oriented and includes only two elementsxe2x80x94the first and second non-adjacent folding optics. The POP is vertically oriented and is established by the true and/or effective centers of the second non-adjacent folding optic, the adjacent folding optic, the near-eye optic and the forward gazing eye.
Note the term xe2x80x9ceffective centersxe2x80x9d refers to the fact that, on one hand, an LDE may be asymmetrical thus making it difficult to define a xe2x80x9ctruexe2x80x9d center; and, on another hand, the illuminated portion of an LDE need not coincide with the general center of the element when the projected area of the LDE is larger than the area of the incident illumination (i.e., the xe2x80x9cilluminated areaxe2x80x9d), provided the illuminated area represents a complete, uncropped version of the image source. With regard to the latter point, in other words, the condition of proper alignment requires that the LDE be of sufficient size that the illuminated area corresponds to an uncropped representation of the image source, the center of which is the xe2x80x9ceffective centerxe2x80x9d.
These and other modifications and applications of the present invention will become apparent to those skilled in the art in light of the following description of embodiments of the invention. However, it is to be understood that the present disclosure of these mechanisms are for purposes of illustrations only and are not to be construed as a limitation of the present invention. All such modifications that do not depart from the spirit of the invention are intended to be included within the scope of the claims and specifications stated within.